The installation and use of cache memory in a computer system is common, since the use of cache memory can enhance computer program execution speed significantly. Further, the use of so-called “redundant array of inexpensive disks” (RAID) is commonly used to store critical data. As the price of RAID storage decreases, its applications increase and it is envisioned that RAID storage may be available to consumers for personal use.
Disk storage devices are mechanical in nature. This can be problematical insofar as a computer system accessing the disk storage faces a bottleneck. Disk input/output is generally slower than microprocessor operations and memory access. In general, a RAID system organizes a plurality of disks to create a virtual disk volume. Further, there is usually a RAID controller that accepts the input/output requests and dispatches it to the disks.
There are several RAID architectures that organize the disks in different manners to provide different advantages. For example, a “RAID-0” system distributes data blocks throughout the disks to expand the storage volume, to balance the loading of each disk, and to enhance throughput. A “RAID-1” system provides a duplicate set of mirrored disks and stores each data block to the paired disk. Such an architecture solves the reliability problem and keeps data even after any one disk failure. The disadvantage of such a system is that it doubles the disk cost.
In yet another RAID system, known as the “RAID-5” system, the data is interleaved blockwise over all of the disks and parity blocks are added and distributed over all the disks. This provides reliability similar to a RAID-1 system and can recover data when a single disk fails by reading the parity block in other data blocks on the same stripe. On the other hand, the RAID-5 architecture provides a larger space than a RAID-1 system because it only uses one disk for the redundant data. Raid-5 systems also balance the loading of the disks by distributing parity blocks over all of the disks. One drawback of the RAID-5 system is that it generates in general more disk input/outputs for each write operation. A write operation to a block of a RAID-5 volume will be dispatched as two read operations and two write operations.